Add virtual Initialize methods to PortAllocator and NetworkManager.

webrtc/p2p/base/portallocator.h

Index: webrtc/p2p/base/portallocator.h
diff --git a/webrtc/p2p/base/portallocator.h b/webrtc/p2p/base/portallocator.h
index 7d2a59f278ccb307ce0266ef89f65234eb291768..6a32b9668f882ddc245f4f2af9bc3683ced930a4 100644
--- a/webrtc/p2p/base/portallocator.h
+++ b/webrtc/p2p/base/portallocator.h
@@ -221,8 +221,12 @@ class PortAllocatorSession : public sigslot::has_slots<> {
   friend class PortAllocator;
 };
 
-// Note that this class should only be used on one thread.
-// This includes calling the destructor.
+// Every method of PortAllocator (including the destructor) must be called on
+// the same thread, except for the constructor which may be called on any
+// thread.
+//
+// This allows constructing a PortAllocator subclass on one thread and
+// passing it into an object that uses it on a different thread.
 class PortAllocator : public sigslot::has_slots<> {
  public:
   PortAllocator() :
@@ -232,10 +236,13 @@ class PortAllocator : public sigslot::has_slots<> {
       step_delay_(kDefaultStepDelay),
       allow_tcp_listen_(true),
       candidate_filter_(CF_ALL) {
-    // This will allow us to have old behavior on non webrtc clients.
   }
   virtual ~PortAllocator() {}
 
+  // This should be called on the PortAllocator's thread before the
+  // PortAllocator is used. Subclasses may override this if necessary.
+  virtual void Initialize() {}
+
   // Set STUN and TURN servers to be used in future sessions, and set
   // candidate pool size, as described in JSEP.
   //